In operation the magnetic field aligns the magnetic moments of the hydrogen nuclei, and the transmitted RF signals re-orient the magnetic moments of the hydrogen nuclei. The rate at which the magnetic moments return to their aligned position provides information about the pores in which the liquids are residing. NMR logging tools are typically operated on wireline in a lower, open hole section of the wellbore.
WO-9803767-A discloses a logging method wherein a string of logging tools is arranged within a lower end part of a casing, referred to as the casing shoe track, to be positioned in the wellbore. The logging tools are operated during lowering of the casing shoe track through the open hole section of the wellbore. The string of logging tools is retrieved to surface after the casing has been installed. A problem of the known method arises if an NMR tool is to be included in the string because in order to achieve adequate magnetic field strength a magnet of relatively large size has to be applied in the tool string which thereby becomes of unacceptably large diameter. Conversely, if a small diameter magnet is used in order to keep the string dimension acceptable, the induced magnetic field is relatively weak.
It is therefore an object of the invention to provide an improved NMR logging assembly which induces a strong magnetic field into the formation and which does not have the limitations referred to above.